If you're a cyclist then you'll surely agree: bike theft is a scourge, and any method the police use to combat it is to be welcomed.

Any method? There is a technique shown to dramatically reduce cycle theft levels, and yet it remains hugely controversial – bait bikes.

This tactic, also known as decoy or tracker bikes, sees police leaving badly locked, or even unlocked, bicycles in vulnerable locations. They are fitted with hidden GPS devices, letting officers trace them to the thieves, or better still to a lock-up or warehouse used by gangs to store lots of stolen bikes.

Police forces around the country are to deploy bait bikes after a series of successful pilot schemes throughout 2008 and 2009, with London's mayor, Boris Johnson, also approving the tactic for the capital earlier this month.

The pilot schemes seemed to show bait bikes are a significant deterrent. Bikes thefts dropped by 45% at Cambridge rail station when British Transport Police tested the method. Even in the UK's bike theft capital, London, rates dropped by around a third in one local trial.

The argument in favour was summed up by Jenny Jones, one of two Greens in the London Assembly:

There are two main deterrents to cycling: road safety and theft. Gangs of young kids often steal the bikes and pass them on to criminals who store them in lock ups and garages before selling them. The advantage of using bait bikes is that it enables you to track the stolen bike back to the organised gang lock up, rather than just arresting the young bike thieves... You've got to get the gangs. You've got to find out where they keep the bikes.

The contrasting view can be summed up in one word: entrapment. Critics argue that particularly when bait bikes are left unlocked they are an open incentive to commit crime, most notably to drug addicts or the young and impulsive.

A drugs treatment adviser to the Home Office, speaking anonymously, described it as "lazy policing", adding:

There is so much else they can do to prevent bike theft that doesn't involve leaving temptation in the way of drug addicts that we have spent months helping to get clean.

In many countries, this practice would be classified as entrapment and would be illegal, the courts would view this as luring people into crime.

Other can fall foul of bait bikes. One Cambridge University student recounted leaving a local nightclub after a few drinks and thinking it would be a good idea to "borrow" an unlocked cycle he spotted nearby. Even though police let him off with a warning, he was perturbed by the tactic:

I think that this is a honeypot trap of the most wasteful kind, and should not be a method of catching the gangs of bike thieves that doubtless exist - it's striking at the bottom rung of the ladder, and this always proves ineffective.